The present invention relates to ink jet printing, and particularly to the characteristics of ink drops ejected from the individual nozzles of an ink jet printhead.
Ink jet printing includes ejecting or jetting drops of liquid ink from selected nozzles of a printhead to form an image on an image receiving surface, such as an intermediate transfer surface, or a media substrate such as paper. Some ink jet printers receive ink in its liquid form. The liquid ink is stored in containers. Other printers receive ink in a solid form.
Solid ink or phase change ink printers conventionally receive ink in a solid form and convert the ink to a liquid form for jetting onto the image receiving surface. The printer receives the solid ink either as pellets or as ink sticks in an ink feed system. With solid ink sticks, the solid ink sticks are fed by gravity, spring force, or other driver through the ink feed system toward a heater plate. The heater plate melts the solid ink into its liquid form. U.S. Pat. No. 6,840,612 for a Guide for Solid Ink Stick Feed issued Jan. 11, 2005, to Jones et al.; U.S. Pat. No. 5,734,402 for a Solid Ink Feed System, issued Mar. 31, 1998 to Rousseau et al.; and U.S. Pat. No. 5,861,903 for an Ink Feed System, issued Jan. 19, 1999, to Crawford et al. describe exemplary systems for delivering solid ink sticks into a phase change ink printer.
The ink feed system delivers the liquid ink to an ink jet printhead. The ink jet printhead contains a plurality of drop generators for ejecting drops of ink onto the image receiving surface. Each drop generator includes an ink conduit leading to an orifice or nozzle through which a drop of ink can be ejected, and an ink drop ejector for causing a drop of ink to be ejected from the ink conduit through the nozzle orifice. Activation signals delivered to each ink drop ejector cause the ejector to eject the drop of ink.
In thermal ink jet printheads, the ink drop ejectors are thermal ejectors that heat ink in the conduit to boil the ink and form a gas bubble behind the drop of ink to be ejected, forcing the drop of ink from the ink jet nozzle orifice. The thermal ejectors heat the ink in response to activation signals received at the thermal ejector.
In piezo-electric ink jet printheads, the ink drop ejectors are piezo-electric ejectors that line the ink conduit near the orifice. The piezo-electric ejectors change shape in response to an electrical activation signal to force a drop of ink from the ink jet nozzle orifice.
Various factors affect the size and trajectory of the ink drops ejected from a printhead nozzle. Among those factors are the size and shape of the nozzle opening, the responsiveness of the ink drop ejectors to particular activation signals, and the magnitude, duration, and shape of the activation signals.
In certain types of printheads, the characteristics of the ink jet drop generators may change over time or usage, so that the size of the ink drop ejected in response to a given activation signal changes over time. Such change in the ink drops may produce undesired change in the image formed on the image receiving surface. Therefore, some printers have included schemes to attempt to compensate for this change in the ink drops. Some ink jet printers incorporate an algorithm to alter the activation signals supplied to the ink drop ejectors as the printhead ages to compensate for anticipated changes to the characteristics of the ink jet drop generators, and to maintain a consistent ink drop size over time. Some printers, such as the Tektronix/Xerox Phaser 840 phase change ink printer, have an algorithm that examines the time and temperature history of the printhead, makes certain assumptions about how the characteristics of the ink jet drop generators are likely to have changed in response to that history, and alters the activation signals supplied to the ink drop ejectors based on those assumptions. Implementing such an algorithm requires an understanding of the relationship between the time and temperature history and changes in the characteristics of the ink jet nozzles.